There are several modes of vehicle operation wherein it is advantageous to turn off or cut the fuel to specific engine cylinders. Typically, fuel cutoff modes can be initiated upon detection of an engine overspeed condition, upon detection of a vehicle overspeed condition, upon detection of a partial or full ignition system failure on a subset of cylinders, or upon detection of a need for a reduction of engine torque, such as for traction control or anti-wheel-spin control purposes.
As the name suggests, during a fuel cutoff mode of operation, fuel is no longer supplied to one or more engine combustion cylinders. Fresh air, however, continues to flow through the cylinders. A problem results when the remaining fueled cylinders are calibrated to run richer-than stoichiometric air/fuel ratio and the engine exhaust temperatures are at or near the design limit for high temperature. In this situation, the fresh air of the deactivated cylinders meets the unburned fuel products of the rich firing cylinders in the catalytic converter. During this interaction, the excess fuel can burn in the presence of the catalyst, causing potentially damaging temperatures.
One way to limit this over-temperature problem is to run the firing cylinders with an air/fuel ratio that is leaner than the stoichiometric ratio, so as to reduce the quantity of unburned fuel products in the catalytic converter. For example, U.S. Pat. No. 4,951,773, issued to Poirier et al., discloses a traction control system fuel control utilizing an air/fuel enleanment strategy. Poirier et al. teaches expressing the enleanment as an adder to the normal air/fuel schedule. Other cylinder cutout strategies are disclosed in U.S. Pat. Nos. 4,489,695 issued to Kohama et al., 4,509,488 issued to Forster et al., and 5,154,151 issued to Bradshaw et al. The problem with these strategies, however, is that the engine exhaust valves have a peak temperature tolerance of around 1650.degree. F., and this temperature can be exceeded with a lean air/fuel during engine operation at high speed/loads.
There is, therefore, a need to develop a strategy to maintain temperatures of the exhaust valves, as well as the catalytic converter, during fuel cutoff modes of operation.